The ZnO varistor is characterized by a quite remarkable nonlinearity of the characteristic voltage-current (V-I) curve. Thus, assuming an equation I=KV.sup..alpha. for the above characteristic curve, the voltage nonlinearity factor .alpha. of this type of varistor is mostly in range from 25 to 50, though sometimes even higher than 50. This favorable nonlinearity characteristic has hitherto been used for surge absorption. As a voltage is applied, however, the ZnO varistor, as characterized above, leaks current. Accordingly, if a voltage is continuously applied thereto for a long time, this voltage-current characteristic property will gradually undergo deterioration. This poses the danger that the varistor may break down and short-circuit resulting in a thermal runaway.
A surge absorber composed of a ZnO varistor and a microgap type surge absorbing element electrically connected in series has been proposed by the inventors (Japanese Unexamined Patent Publication No. 58-95933).
This permitted the above ZnO varistor to stay ready to work with a high surge response characteristic of its own while suppressing the aforementioned leak current to a minimum level. This prevents the deterioration thereof and avoids the accompanying danger as mentioned above.
However, the proposed surge absorber uses a ZnO varistor which is characterized by a large voltage nonlinearity factor .alpha. and thus selected among those products having a varistor voltage higher than the line voltage.
As mentioned above, the proposed surge absorber uses a ZnO varistor having a varistor voltage higher than the line voltage, so the above surge absorber is characterized by a high firing potential and surge response voltage. This necessarily results in a slow surge response. Accordingly, the previously proposed surge absorber has a low absorbing ability and an insufficient surge absorption capacity for protection of devices.